The present application relates generally to pressure indicating devices. More particularly, it relates to a device with a tapered spiral bellows pressure sensor element, where the pressure to be measured causes a predominantly axial displacement of the bellows, which can be observed and correlated to the amount of pressure.
Many instruments for measuring pressure rely on the pressure causing a mechanical deformation of a resilient mechanical structure such as bellows. The deformation of the structure causes an indicator to move, allowing a user to observe a value for the amount of pressure. In some fields, such as medical devices or fields in which an extreme purity of gases must be maintained, it is important to be able to measure pressure with a single element that can be made from a material compatible with the application.
Bellows can be used to measure pressure through visual monitoring of the extension of the bellows. However, bellows are difficult to manufacture with walls thin enough for accurately measuring very low pressures. Moreover, variations in wall thickness during manufacturing can lead to significant variations in accuracy. For instance, a 10% variation in wall thickness of conventional cylindrical bellows can lead to 20% or more variation in pressure indicated depending on the design of the bellows' convolutions. One of the causes of wall thickness variation is the difficulty with which the convolutions are made; most bellows are made using blow molding or injection molding, which often does not lead to accurate wall thickness, particularly in corners. Errors in thickness in the corners leads to variation in effective diameter. The diameter of a plate, which the convolution effectively is with regard to predicting compliance, varies with the diameter squared. Material thickness variation in the acute angle of the corners where two convolutions meet is typically much greater than the thickness variation on the plate region; hence it causes a more significant variation in the effective plate diameter and hence compliance of the bellows. This makes accurate measurement of very small pressures, e.g., on the order of millibar to centibars, very difficult.
When a bellows is injection molded, greater thickness control can be achieved. However, removing bellows from molds without breaking the bellows can be difficult. Additionally, it is difficult to make bellows thin enough using molding for accurate low pressure measurement.
A further difficulty with bellows is the potential for sliding mechanical contact with the supporting instrument body, and friction is notoriously non-repeatable, which gives further rise to variations in pressure measurement readings, particularly at low pressures.